Respiration and metabolism · GCSE Biology

Aerobic respiration

Respiration is an exothermic reaction that happens continuously in living cells to release energy for their processes.
Aerobic respiration uses oxygen and happens mostly in mitochondria.
Word equation: glucose + oxygen → carbon dioxide + water.
Symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
Organisms need energy for chemical reactions that build larger molecules, for movement and, in mammals and birds, to keep warm.

Anaerobic respiration happens without oxygen and releases much less energy than aerobic respiration, because glucose is only partly broken down.
In muscles: glucose → lactic acid.
In plant and yeast cells: glucose → ethanol + carbon dioxide. In yeast this is called fermentation.
Fermentation is used to make bread (the CO₂ makes dough rise) and alcoholic drinks (ethanol).

During exercise, heart rate, breathing rate and breath volume increase to supply the muscles with more oxygenated blood and remove carbon dioxide.
If muscles don't get enough oxygen, they respire anaerobically and lactic acid builds up. Muscles become fatigued and stop contracting efficiently.
The oxygen debt is the amount of extra oxygen the body needs after exercise to react with the lactic acid and remove it from the cells.
(Higher) Blood carries lactic acid to the liver, where it is converted back into glucose.

Metabolism is the sum of all the reactions in a cell or the body. Energy from respiration is used for many of them.
Examples: glucose is converted to starch, glycogen and cellulose; one glycerol and three fatty acids form a lipid molecule.
Glucose and nitrate ions make amino acids, which are used to make proteins.
Excess proteins are broken down to form urea for excretion.

Respiration: An exothermic reaction in cells that releases energy from glucose.
Aerobic respiration: Respiration using oxygen: glucose + oxygen → carbon dioxide + water.
Anaerobic respiration: Respiration without oxygen, releasing less energy.
Lactic acid: The product of anaerobic respiration in muscles.
Fermentation: Anaerobic respiration in yeast, producing ethanol and carbon dioxide.
Oxygen debt: The extra oxygen needed after exercise to react with lactic acid and remove it from cells.
Exothermic: A reaction that transfers energy to the surroundings.
Metabolism: The sum of all the reactions in a cell or the body.
Mitochondria: The organelles where aerobic respiration happens.
Glycogen: A carbohydrate that stores glucose in the liver and muscles.